Method For Assigning Scrambling Codes In A Cdma Cellular Mobile Communications Network

ABSTRACT

A method of assigning scrambling codes to cells of a CDMA cellular mobile communications network includes assigning to each cell of a set of cells of a network area a cell attribute adapted to identify a cell typology, and, for each cell, assigning a respective primary scrambling code according to the cell typology, exploiting said cell attribute. The cell attribute includes a first attribute value adapted to identify a cell in which a complexity of computation to be performed by the user equipment for determining the cell primary scrambling code is to be minimized, and a second attribute value adapted to identify a cell in which a synchronization time of mobile communications user equipment is to be minimized. If the cell has an attribute value equal to said first attribute, the cell is assigned a primary scrambling code different from the primary scrambling codes assigned to neighboring cells. If the cell has an attribute value equal to said second attribute, the cell is assigned a primary scrambling code not belonging to a code group to which a primary scrambling code assigned to a neighboring cell belongs.

FIELD OF THE INVENTION

The present invention generally relates to the field oftelecommunications, particularly to mobile communications networks,allowing communications between mobile users. More specifically, theinvention relates to cellular mobile communications networks that adopta Code Division Multiple Access (CDMA) access scheme, such as cellularnetworks of the third generation.

BACKGROUND OF THE INVENTION

A peculiarity of cellular mobile communications networks is that theyfeature a plurality of so-called “network cells”, where by the term cellthere is intended the set of geographical points (“pixels”, in jargon)covered by the radio electromagnetic signal irradiated by a commonantenna. Cellular mobile communications networks thus provide coverageof a determined geographic region by means of the plurality of networkcells.

Among the different type of cellular mobile communications networks,some networks have a radio access front end exploiting the CDMA accessscheme to the shared (radio) communication medium. This is for examplethe case of third-generation cellular mobile communications networks,currently starting to be deployed. One of the third-generation mobilecommunications standards is the so-called Universal MobileTelecommunications System (UMTS), which is in particular the standardthat has been adopted by operators in Europe.

CDMA is a technique of accessing a shared communications mediumaccording to which a same frequency bandwidth (a “channel”) is assignedsimultaneously to all the requesting users; the discrimination amongdifferent signals intended for different users is accomplished byexploiting a coding scheme, according to which different codes areassigned to different users, and the signals directed thereto are codedusing the respective codes. The codes assigned to different users andexploited for coding the signals directed thereto need to be“orthogonal”. The coding process comprises a so-called “spreading”operation, according to which the bandwidth of the original (uncoded)signal is widened, in particular spread over a larger bandwidth, at thesame time reducing the average signal power. The spreading is achievedby coding the signal using a code that contains a higher number ofsymbols than the number of bits to be transmitted; the coded signal thushas a symbol rate (the so-called “chip rate”) higher than the bit rateof the original signal.

A “scrambling” process is further implemented, by applying a “scramblingcode” to the signal after the spreading operation, with the purpose ofscrambling the different symbols. The scrambling operation does notincrease the signal bandwidth (the symbol rate is not changed comparedto the chip rate of the spread signal), and can be viewed as theaddition of a “color” to the signal, that allows identifying thetransmission source. Particularly, in downlink (i.e., from the radiobase station to the mobile terminal), the scrambling process allowsdistinguishing the signals within a given network cell from the signalwithin a different cell: to this purpose, different scrambling codes areused in different cells, in particular if such cells are neighboring.

The adoption of the CDMA access scheme has an impact on the “handover”procedures, by which, generally speaking, there is intended the set ofprocedures that makes it possible to keep active a service provided to ageneric mobile user even when the user moves. In particular, in the CDMAaccess scheme a mobile user may exploit a same radio channel indifferent cells; thus, the passage of responsibility (handover) of agiven mobile user from one network cell to another adjacent thereto(typically, in consequence of the movement of the mobile user throughthe geographic area) can be handled by keeping the communication withthe user active on the same channel. In particular, thanks to the factthat the signals issued from different sources (different antennascorresponding to different cells) are distinguishable because of the useof different scrambling codes, a mechanism referred to as“soft-handover” (relying on a particular type of receiver in the UserEquipment—UE—, called “Rake”) allows the mobile user's terminal todecode signals coming from, and thus to exchange information with, twoor more different antennas or, more generally, with different radio basestations. In particular, thanks to this “soft-handover” mechanism, theUE can distinguish between signals issued by different cell radio basestations by looking at the different signal color. This takes place inparticular areas, referred to as “soft-handover areas” or“macrodiversity areas”. The different network cells to which the UE issimultaneously connected form the so-called “active set”.

As known, in the UMTS the set of scrambling codes used in downlink isrepresented by the Gold codes featuring low self-correlation andcross-correlation. The length of the Gold code for the UMTS system is inprinciple equal to eighteen bits, for a total of 2¹⁸−1=262,143 differentcodes. However, in order to keep the receiver not too complex, only afraction of such a vast set of codes is effectively exploited inpractical implementations. Specifically, the standard prescribes thatthe number of usable codes in UMTS networks is limited to a pool of8,192 different codes. The pool of 8,192 usable scrambling codes issubdivided into 512 groups, each group including 16 codes, where one ofthe sixteen codes takes the role of a so-called “primary scramblingcode”, and the remaining 15 codes of the group are “secondary scramblingcodes”.

When planning an UMTS network, or a particular regional area thereof, aunique primary scrambling code (and, consequently, the associated 15secondary scrambling codes associated to that primary scrambling code),chosen among the available 512 primary scrambling codes, has to beassigned to each cell of the area under planning.

The pool of 8,192 scrambling codes available for use is furtherconsidered as subdivided into 64 code groups of 128 codes each, where,within the generic code group, eight codes among the 128 codes areprimary scrambling codes; thus, the pool of 8,192 available scramblingcodes includes 64 code groups, each one including eight primaryscrambling codes (and associated secondary scrambling codes).

In downlink the primary scrambling code plays a role in the procedurecalled “cell search”, which includes the set of operations that allowthe UE synchronize to the network and decode the control channels of thenetwork cell wherein it is located. Specifically, the UE invokes thecell search procedure in either one of two cases:

whenever the UE is turned on and has to register to the network for thefirst time (after a previous de-registration in consequence to a turnoff); or

for purposes of measuring the common channels of the adjacent cells,with the aim of updating the so-called “active set” of different networkcells to which the mobile terminal is connected (a procedure called“cell reselection”).

The cell search procedure impacts the UE performance: depending on thecomplexity of the operations to be performed, the UE battery chargeconsumption, as well as the time required by the UE for synchronizingand decoding the control channel (the so-called “Broadcast ControlChannel”—BCH) over which the network information travels vary. Inparticular, the cell search procedure impact on the battery chargeconsumption is higher when the procedure is performed in support of thecell reselection procedure, because such operation is carried out morefrequently compared to the initial synchronization of the UE uponturning it on.

The assignment of scrambling codes in downlink can be effected by meansof planning algorithms, such as the one described in Y. H. Jung and Y.H. Lee, “Scrambling code planning for 3GPP W-CDMA systems”, IEEE VTC2001Spring, Rhodes, Greece, May 2001.

In particular, the scrambling code assignment has to satisfy a primaryscrambling code re-use requirement, according to which unique primaryscrambling codes, within the set of 512 available primary scramblingcodes, have to be univocally assigned to neighboring cells belonging tothe geographic area being planned. In particular, two generic cells areconsidered neighboring cells if at least one pixel of a cell “touches” apixel of the other cell (geometrical neighborhood), or, according to adifferent criterion, if there exist portions of a cell wherein the powerlevel of a predetermined channel, particularly the CPICH (RSCP) pilotchannel of the other cell exceeds a predetermined threshold(electromagnetic neighborhood).

As it can be derived from the discussion made in S. Kourtis, “CodePlanning Strategy for UMTS-FDD networks”, in Proceedings of VTC 2000,Tokio, Spring 2000, the distance between primary scrambling codesassigned to neighboring network cells impacts the performance of the UE,directly affecting the computational cost of the cell search procedure,and the synchronization time of the UE, at the frame level. Inparticular, with the increase of the number of scrambling codes groupsassigned to adjacent cells, within the pool of 512 scrambling codes, thetime needed for achieving synchronization rises, whereas thecomputational complexity is reduced. On the contrary, with the decreaseof the number of scrambling codes groups assigned to adjacent cells,within the pool of 512 primary scrambling codes, the time needed forachieving synchronization decreases, but the computational complexityincreases.

The choice of which strategy to adopt is not univocal, and depends onthe considered scenario. According to Kourtis, in a case of a micro-cellnetwork (i.e., a network having cells of limited area coverage), typicalof a urban environment, it is important to minimize the time required bythe cell search procedure, particularly if the UEs exhibit a medium/highmobility: a poor synchronization performance means that the number ofmeasurements taken by the UE per measuring period is small, thus the UEmay not have the necessary information required to perform thesoft-handover efficiently.

In the case of a macro-cell network (with network cells of relativelywide area coverage), more typical of sub-urban and rural environments,the greater cells' dimension allows tolerating higher delays in the cellreselection procedure. Then, it is preferable to privilege strategiesthat minimize the computational cost of the mobile terminal, thusincreasing the battery charge life, also in view of the fact that, onaverage, the transmitted power of the terminals is higher in macro-cellenvironments than in micro-cell ones.

Kourtis provides guidelines for planning the primary scrambling codes indownlink; in particular, the guidelines are given providing some primaryscrambling codes planning configurations which are to be preferred, andwhich are expressed in terms of the number (M) of different code groupsamong which the primary scrambling codes to be assigned to neighboringnetwork cells are chosen, and the number (L) of primary scrambling codeswithin each code group.

SUMMARY OF THE INVENTION

The Applicant has observed that the approach outlined by Jung and Leefinds a limitation in the fact that the scrambling codes assignmentstrategy does not take in consideration the cell's typology.

As it can be derived by the discussion made by Kourtis, given twoneighboring cells in the set of cells being planned, where byneighborhood there is intended either geometrical or electromagneticneighborhood, or both, and provided that the primary code re-userequirement is satisfied, the assignment of the scrambling codes maynevertheless lead to substantial differences in the mobile terminals'performance, in dependence of the distance between the codes assigned tothe two cells and of the cells' typology.

On the other hand, the Applicant observes that the guidelines set forthby Kourtis cannot be applied in general, because in some cases, such asfor example in the case of the UMTS standard, the number (L) of primaryscrambling codes within each code group is not variable, rather fixedand equal to 8. Moreover, in a real UMTS network the size of the set ofcells monitored by the mobile terminal during the measurement operationscarried out for the cell reselection procedure cannot be defined apriori, and thus the lower limit (equal to 12) set by Kourtis does notapply.

In view of the outlined state of the art and related problems, drawbacksand limitations, the Applicant has tackled the problem of how to performscrambling code planning, i.e. how to assign scrambling codes, indownlink, to the cells of a CDMA-based mobile communications network,particularly a UMTS network, in a way that takes into account thenetwork's cell typology and the peculiarities of the primary scramblingcodes in the UMTS system, so as to substantially improve the UEs'performance trading off between battery charge consumption andsynchronization speed.

According to an aspect of the present invention, there is provided amethod of assigning scrambling codes to cells of a CDMA cellular mobilecommunications network as set forth in the appended claim 1. The methodcomprises assigning to each cell of a set of cells of a network area acell attribute identifying a cell typology; and assigning to each cellof said set of cells a respective primary scrambling code according tosaid cell attribute.

In particular, for each cell, a set of neighboring cells is defined,according to a predetermined neighborhood criterion, which may forexample include a geometrical neighborhood criterion, or anelectro-magnetic neighborhood criterion.

In an embodiment of the present invention, said cell attribute comprisesa first attribute value adapted to identify a cell in which a complexityof computation to be performed by the user equipment for determining thecell primary scrambling code is to be minimized, and a second attributevalue adapted to identify a cell in which a synchronization time of amobile communications user equipment is to be minimized.

In an embodiment of the present invention, if the cell has an, attributevalue equal to said first attribute value, the method comprisesassigning to the cell a primary scrambling code different from theprimary scrambling codes assigned to the neighboring cells.

In an embodiment of the present invention, if the cell has an attributevalue equal to said second attribute, the method comprises assigning tothe cell a primary scrambling code belonging to a code group differentfrom the code groups to which the primary scrambling codes assigned tothe neighboring cells belong.

The method may further comprises assigning to the cell a primaryscrambling code different from the primary scrambling codes assigned tothe neighboring cells in case a primary scrambling code belonging to acode group different from the code groups to which the primaryscrambling codes assigned to the neighboring cells belong cannot befound.

Another aspect of the present invention concerns a method of planning aCDMA cellular mobile communications network as set forth in appendedclaim 9, comprising a method of assigning scrambling codes to cells of aCDMA cellular mobile communications network as defined above.

The method can be implemented in software, and be implemented by a dataprocessing apparatus. Thus, still other aspects of the present inventionconcerns a computer program directly loadable into a working memory of adata processing system and adapted to implement, when executed, themethod defined above, and a computer program product comprising, storedon a computer readable media, the said computer program.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be madeapparent by the following detailed description of an embodiment thereof,provided merely by way of non-limitative example, description that willbe conducted making reference to the annexed drawings, wherein:

FIG. 1 pictorially shows a portion of a UMTS network being planned,intended to cover a respective geographic area, with a plurality ofnetwork cells to which scrambling codes are to be assigned during theplanning process;

FIG. 2 schematically represents a spreading and scrambling operationsperformed on a signal to be transmitted, according to the UMTS standard;

FIG. 3 schematically shows, in terms of a timing diagram, the result ofthe operations of spreading and scrambling on a signal to betransmitted;

FIG. 4 pictorially shows a subdivision in groups of the scramblingcodes, according to the UMTS standard; and

FIG. 5 is a schematic flowchart showing the main steps of a scramblingcodes assignment method according to an embodiment of the presentinvention, for assigning scrambling codes to the cells of the networkarea being planned;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

Referring to the drawings, in FIG. 1 there is schematically depicted aportion of UMTS network under planning, particularly an UMTS networkportion intended to provide cellular mobile communications capabilitywithin a given geographic area.

The UMTS network portion under consideration, identified globally byreference numeral 100, comprises a plurality of cells C1, C2, C3, C4, .. . , Cn, each one having a relatively limited area coverage (the areacoverage of the generic cell depending on several factors such as, forexample, the environment wherein the UMTS network is installed, as willbe described in greater detail later on). The cells are each one made upof a plurality of pixels, i.e., they are the set of geographic pointscovered and served by the radio electromagnetic signal irradiated by arespective cell's antenna.

Usually, groups of three to six cells (on average) are managed by anetwork entity called “Node B”, such as the Node Bs 105 a and 105 b inthe drawing (where, merely by way of example, it is assumed that thecells C1, C2, C3, and C4, are managed by the Node B 105 a, and that thecell Cn is managed by the Node B 105 b).

Groups of Node Bs are managed by a Radio Network Controller (RNC), likethe RNC 110 shown in the drawing; the RNCs are connected to the coreUMTS network 115.

In the drawing, the cells C1, C2, C3, C4, . . . , Cn are depicted aspartially overlapping, in order to schematize a concept of “cellneighborhood”. Neighborhood can be defined adopting different criteria:for example, as mentioned in the foregoing, two generic cells may beconsidered to be neighboring if at least one pixel of a one cell“touches” a pixel of another cell (geometrical neighborhood);alternatively, two generic cells may be considered to be neighboring ifportions of a cell exist wherein the power level of a predeterminedcommunications channel, for example the Control PIlot CHannel CPICH(RSCP) pilot channel of another cell exceeds a predetermined threshold(electromagnetic neighborhood).

A generic UE, like the UE 125 depicted as located within the cell C1,can thus receive signals transmitted by the antennas of different cells,and is able to differentiate among them.

This differentiation of signals coming from different transmitters ismade possible by the adoption of the CDMA access technique. Makingreference to FIGS. 2 and 3, each transmitter (particularly, thetransmitter of the generic network cell) implements a spreading and asubsequent scrambling of the signal to be transmitted. The signal (data)to be transmitted 205, having a given symbol rate (usually called thebit rate) is first submitted to a spreading process 210, using aspreading code 215, in order to “widen” its spectrum and distribute (andthus lower) its power over the whole channel bandwidth. The spreadingcode 215 has a higher number of symbols than the signal to betransmitted 205, thus the spread signal 220 has a symbol rate (a chiprate, in jargon) higher than the bit rate of the initial signal 205.

The spread signal is then submitted to a scrambling process 225, using aspreading code or scrambling sequence 230. The signal scrambling processdoes not change the chip rate, thus the signal 235 to be transmitted“over-the-air” has a chip rate equal to that of the spread signal.

The scrambling process is used for making signals transmitted by thetransmitters of different cells distinguishable.

To this purpose, every cell has to use a unique scrambling code, andthus a problem of scrambling code assignment rises.

As mentioned in the foregoing, in the UMTS standard the set ofscrambling codes used in downlink is represented by the Gold codesfeaturing low self-correlation and cross-correlation. The length of theGold code for the UMTS system is equal to eighteen bits, thus, inprinciple a total of 2¹⁸−1=262,143 different codes would be available.However, in order to keep the receiver (particularly, the one on-boardthe UEs) not too complex, only a fraction of such a vast set of codes iseffectively exploited. Specifically, the standard prescribes that thenumber of usable codes in UMTS networks is 8,192.

Making reference to FIG. 4, the pool of 8,192 usable scrambling codes(identified globally by 400 in the drawing) is subdivided into 64groups, like the group GR1 in the drawing, wherein each of saidscrambling codes group, as depicted in the drawing, contains 128different scrambling codes.

Within the 8,192 different scrambling codes, 512 of them are selectedand adopted as “primary scrambling codes” PSC, whereas the remainingscrambling codes are “secondary scrambling codes” SSC. This simplifiesthe procedures of the search of the scrambling code and cellidentification by the UE: in fact, some of the channels (for example,the Common Control Physical CHannel—CCPCH—and the Common PIlotCHannel—CPICH) always use the primary scrambling code, whereas otherphysical channels in downlink may either use the primary scrambling codeor a secondary scrambling code.

By defining the 512 primary scrambling codes, the pool of 8,192different scrambling codes can be viewed as subdivided into 512 sets of16 scrambling codes each, where one of the sixteen scrambling codes is aprimary scrambling code, the remaining 15 being secondary scramblingcodes.

Therefore, referring again to FIG. 4, each group of 128 scramblingcodes, like the group GR1, includes eight sets of 16 scrambling codes,and thus eight primary scrambling codes (those depicted as shadedsquares), with the associated secondary scrambling codes.

The geographic area represented schematically in FIG. 1 is assumed to bean area of the UMTS network to be planned. Planning the network areameans, among other issues, assigning (primary) scrambling codes to thedifferent cells of the area under planning.

In the following, a method according to an embodiment of the presentinvention is described, adapted to be used in the UMTS network planningprocess, for assigning the scrambling codes to the different networkcells of the area under planning. Reference will be made to theschematic flowchart of FIG. 5.

Firstly, a set

-   -   C={C1, C2, . . . , Cn}, with n denoting a positive integer        is created, where C indicates the set of n cells C1, C2, . . . ,        Cn belonging to the network area being planned, and to which it        is necessary to assign the respective primary scrambling code        (block 505). The number n of cells, and their nature,        particularly their area coverage, is assumed to have been        defined in advance, according to criteria such as the traffic        load to be managed, the traffic geographical distribution, the        network accessibility target (e.g., indoor, outdoor, “in car”, .        . . ), the traffic mobility (e.g. the average speed of the        users), the area targets to be served, etc.

Then, attributes are assigned to each cell in the set C (block 510). Tothis purpose, let

I={I1,I2, . . . , In}

denote a set of cell indicators I1, I2, . . . , In, each cell indicatorbeing related to a respective cell C1, C2, . . . Cn; according to anembodiment of the present invention, the generic cell indicator Ik cantake one of two possible “values”: a value MACRO and a value MICRO.Specifically, the generic indicator Ik takes the value MACRO in case thetypology of the respective cell Ck is such that the assignment of thescrambling code to the respective cell Ck has to be carried outminimizing the computational complexity required to the generic UE, evenif this penalizes the synchronization speed. On the contrary, the cellindicator Ik takes the value MICRO in case the cell's typology is suchthat the assignment of the scrambling code to the respective cell Ck hasto be carried out minimizing synchronization time, penalizing thecomputational complexity required to the generic mobile terminal. Forexample, a generic network cell is characterized by a MICRO attribute incase it belongs to a micro-cellular network environment, such as indense, urban environments; differently, a network cell is characterizedby an attribute MACRO in case it belongs to a macro-cellularenvironment, like in a suburban/rural environment.

Moreover, for the generic cell Ck of the set C, a set A¹k of neighboringcells is defined (block 515), where any other cell of the set C isconsidered neighboring to the cell Ck based on a predeterminedneighborhood criterion, for example the geometrical neighborhoodcriterion or the electromagnetic neighborhood criterion defined in theforegoing (it is however pointed out that different neighboring criteriacan be adopted).

Thus, the set C of cells can be conveniently represented as:

C={C1(I1,A ¹1),C2(I2,A ¹2), . . . , Cn(In,A ¹ n)}.

Moreover (block 520), the pool P of m primary scrambling codes PSC1,PSC2, . . . , PSCm available for use in the area under planning isdefined:

P={PSC1,PSC2, . . . PSCm},

subdivided in groups as depicted in FIG. 4, where the pool P may includeall the available 512 primary scrambling codes within the total of 8,192scrambling code, or only a subset thereof.

According to an embodiment of the present invention, the set C of cellsin the area under planning is then sorted by considering the number ofneighboring cells of the generic cell, as indicated by the cardinalityof the sets A¹ 1, A¹ 2, . . . , A¹n (block 525). This means that, in thesubsequent steps of the method, the cells having a higher number ofneighbors are processed first.

Than, constrains to be respected in the primary scrambling codeassignment procedure are set (block 530). In particular, according to anembodiment of the present invention, two constraints are set:

a first constraint (CONSTRAINT A) preventing the assignment of sameprimary scrambling codes to neighboring cells (this constraintcorrespond to the main, primary scrambling code re-use constraint);

-   -   a second constraint (CONSTRAINT B) according to which primary        scrambling codes belonging to a same group (like the group GR1        in FIG. 4) of scrambling codes are to be assigned to neighboring        cells.

According to an embodiment of the present invention, the secondconstraint (CONSTRAINT B) is susceptible of being relaxed (to the lessstringent first constraint CONSTRAINT A) in case it is of prejudice tothe possibility of assigning the primary scrambling codes to the cells;differently, the first constraint, corresponding to the above-mentionedprimary scrambling code re-use constraint, is a mandatory requirementand can in no case be relaxed.

A range [min, max] of usable primary scrambling codes is then defined(block 535), within the pool of primary scrambling code (possibly, allthe 512 scrambling codes).

A repetitive procedure is then performed: a loop counter i isinitialized to “1” (block 540), and the first cell in the (sorted) set Cis selected for being processed (block 545).

For the selected cell Ci, the set of requirements to be respected in theprimary scrambling code assignment procedure is initialized. Inparticular, it is first ascertained whether the attribute Ii for theselected Ci is MICRO (decision block 550). In the affirmative case (exitbranch Y of decision block 550), meaning that the cell Ci has a limitedarea coverage, as is typical of, e.g. dense, urban environments, or ingeneral any micro-cellular environment, the set of requirements isinitialized to {A,B} (block 555), i.e., the primary scrambling codeassignment procedure will try to assign to neighboring cells primaryscrambling codes of the same group. If instead it is ascertained thatthe attribute Ii for the selected cell Ci is MACRO (exit branch N ofdecision block 550), meaning that the cell has a relatively wide areacoverage, as is typical of, e.g. sub-urban or rural environments, or ingeneral any macro-cellular environment, the set of requirements isinitialized to {A} (block 560), i.e., the primary scrambling codeassignment procedure will respect only the primary scrambling codere-use constraint, avoiding to assign a same primary scrambling code toneighboring cells.

Then, for the selected cell Ci a binary vector

TAVi=[a_(min),a₁,a₂, . . . , a_(max)], with a_(k)=0 or 1

is built, depending on the set of requirements. In particular, if it isascertained that the set of requirements is {A} (exit branch Y ofdecision block 565), the binary vector TAVi is such that (block 570):

-   -   a_(k)=1 if there exist a neighboring cell of the selected cell        Ci (i.e., a cell belonging to the set A¹i) to which the primary        scrambling code PSC_(k) has been assigned; whereas

a_(k)=0 otherwise.

If instead it is ascertained that, for the selected cell Ci, the set ofrequirements is {A,B} (exit branch N of decision block 565), the binaryvector TAVi is built in such a way that (block 575):

a_(k)=1 if there exist a neighboring cell of the selected cell Ci (i.e.,a cell belonging to the set A¹i) to which a primary scrambling codebelonging to the group GRk of primary scrambling codes including thecode PSC_(k) has been assigned; whereas

a_(k)=0 otherwise.

After having built the proper binary vector TAVi, the vector is scanned,starting from the vector component a_(min) that corresponds to the lowerprimary scrambling code in the usable range [min,max], in search of thefirst encountered binary vector component a_(k) that is equal to “0”(block 580).

When such a component a_(k)=“0” is encountered (exit branch Y ofdecision block 583), the corresponding primary scrambling code PSC_(k)(together with the associated set of 15 secondary scrambling codes) isassigned to the cell Ci (block 585).

The loop counter i is increased by one (block 587), and the operationflow jumps back to block 545 (connector J2); a new cell Ci is selectedand processed in the way described above, unless all the cells of theset C have already been processed (decision block 589), in which casethe procedure ends.

If on the contrary no component a_(k)=“0” is found (exit branch N ofdecision block 583), the constraints for the primary scrambling codeassignment-procedure are relaxed (block 591). In particular, relaxingthe constraints means that if the current constraints set is {A,B}, theconstraint B is eliminated, and the constraints set is declassed to {A};if instead the current constraints set is {A}, it is declassed to thevoid set {Ø}.

Then, the operation flow jumps back to block 565 (connector J1), unlessit is ascertained that the updated constraints set is the void set(decision block 593), in which case it is decreed that the assignment ofscrambling code to the cell Ci is impossible because the basic re-useconstraint cannot be satisfied (block 595). The procedure then ends.

For example, if the constraint is relaxed from {A,B} to (A) (whichhappens if a primary scrambling code belonging to a code group differentfrom the code groups to which the primary scrambling codes assigned tothe neighboring cells belong cannot be found), the algorithm assigns tothe cell a primary scrambling code different from the primary scramblingcodes assigned to the neighboring cells, independently from the codegroup.

At the end of the procedure, a respective primary scrambling code (andthe associated secondary scrambling codes) is assigned to each cell inthe area under planning, unless the procedure ended for impossibility ofassigning the scrambling codes satisfying the re-use requirement.

In case of impossibility of assigning to one or more cells thescrambling codes satisfying the re-use requirement, the algorithm mayassign to these cells a particular code external to the set of codespreviously considered.

Thanks to the present invention, a particular embodiment of which hasbeen presented in the foregoing, it is possible to allocate primaryscrambling codes (in downlink) to the network cells taking into accountconstraints related to the kind of environment to which the generic cellbelong (e.g., a micro-cellular environment or a macro-cellularenvironment), and to optimize the scrambling code assignment, optimizingthe trade-off between the minimization of the synchronization timesnecessary to the terminal in urban/dense urban environments and/ormicro-cellular (cells qualified as MICRO), and the minimization of thecomputation complexity posed on the terminal in suburban/rural and/ormacro-cellular environments (cells qualified as MACRO).

The method according to the present invention can be implemented by wayof a program executed by a suitable data processing apparatus, such as apersonal computer or a workstation.

Although the present invention has been disclosed and described by wayof some embodiments, it is apparent to those skilled in the art thatseveral modifications to the described embodiments, as well as otherembodiments of the present invention are possible without departing fromthe spirit or essential features thereof/the scope thereof as defined inthe appended claims.

1-11. (canceled)
 12. A method of assigning scrambling codes to cells ofa CDMA cellular mobile communications network, comprising: assigning toeach cell of a set of cells of a network area, a cell attributeidentifying a cell typology; and assigning to each cell of said set ofcells a respective primary scrambling code according to said cellattribute.
 13. The method according to claim 12, further comprising:defining a set of neighboring cells according to a predeterminedneighborhood criterion for each cell.
 14. The method according to claim13, wherein said neighborhood criterion comprises a geometricalneighborhood criterion.
 15. The method according to claim 13, whereinsaid neighborhood criterion comprises an electro-magnetic neighborhoodcriterion.
 16. The method according to claim 13, wherein said cellattribute comprises a first attribute value adapted to identify a cellin which a complexity of computation to be performed by user equipmentfor determining the cell primary scrambling code is to be minimized, anda second attribute value adapted to identify a cell in which asynchronization time of mobile communications user equipment is to beminimized.
 17. The method according to claim 16, wherein assigning toeach cell of said set of cells a respective primary scrambling codecomprises: if the cell has an attribute value equal to said firstattribute value, assigning to the cell a primary scrambling codedifferent from the primary scrambling codes assigned to the neighboringcells.
 18. The method according to claim 16, wherein assigning to eachcell of said set of cells a respective primary scrambling codecomprises: if the cell has an attribute value equal to said secondattribute, assigning to the cell a primary scrambling code belonging toa code group different from the code groups to which the primaryscrambling codes assigned to the neighboring cells belong.
 19. Themethod according to claim 18, further comprising: in case a primaryscrambling code belonging to a code group different from the code groupsto which the primary scrambling codes assigned to the neighboring cellsbelong cannot be found, assigning to the cell a primary scrambling codedifferent from the primary scrambling codes assigned to the neighboringcells.
 20. A method of planning a CDMA cellular mobile communicationsnetwork, comprising a method of assigning scrambling codes to cells of aCDMA cellular mobile communications network according to claim
 12. 21. Acomputer program directly loadable into a working memory of a dataprocessing system capable of being adapted to implement, when executed,the method according to claim
 12. 22. A computer program productcomprising, stored on a computer readable media, the computer program ofclaim 21.